>> we are reaching out. canada offered up five of their crews today. >> reporter: more than 2,000 firefighters are here, but they are losing ground. even the air attacks can't slow this blaze. >> if you watch those trees spontaneously burst, it's not 1, it's 10, 15 of them, it is pretty frightening. >> reporter: we found strike team charlie, the tactical unit of 16, trying to defend their hometown. when conditions turn in firefighters' favor, like the wind right now, they are able to set backfires to make this blaze close in on itself to suffocate itself out. the work shift will last 36 hours, then a break where they will sleep in the dirt. >>you get pretty tired, but it's what you have to do. >> reporter: safety first, this is dangerous, even deadly work. >> why didn't you put firefighters down there and i say, why didn't you put your children down there? would you? >> reporter: evacuations continue in towns near yosemite national park. 4500 homes are threatened.